Insular Celtic languages are those Celtic languages that originated in Britain and Ireland, in contrast to the Continental Celtic languages of mainland Europe and Anatolia. All surviving Celtic languages are from the Insular Celtic group, including that which is now spoken in Continental Europe; the Continental Celtic languages are extinct. The six Insular Celtic languages of modern times can be divided into:

The proponents of the Insular Celtic hypothesis (such as Cowgill 1975; McCone 1991, 1992; and Schrijver 1995) point to shared innovations among Insular Celtic languages, including inflected prepositions, shared use of certain verbal particles, VSO word order, and the differentiation of absolute and conjunct verb endings as found extensively in Old Irish and to a small extent in Middle Welsh (see Morphology of the Proto-Celtic language). They assert that a partition that lumps the Brittonic languages and Gaulish (P-Celtic) on one side and the Goidelic languages with Celtiberian (Q-Celtic) on the other may be a superficial one (i.e. owing to a language contact phenomenon), as the identical sound shift (/kʷ/ to /p/) could have occurred independently in the predecessors of Gaulish and Brittonic, or have spread through language contact between those two groups.

In order to show that shared innovations are from a common descent it is necessary that they do not arise because of language contact after initial separation. A language area can result from widespread bilingualism, perhaps because of exogamy, and absence of sharp sociolinguistic division.

Ranko Matasović has provided a list of changes which affected both branches of Insular Celtic but for which there is no evidence that they should be dated to a putative Proto-Insular Celtic period.[2] These are:

The Insular Celtic verb shows a peculiar feature unknown in any other attested Indo-European language: verbs have different conjugational forms depending on whether they appear in absolute initial position in the sentence (Insular Celtic having verb–subject–object or VSO word order) or whether they are preceded by a preverbal particle. The situation is most robustly attested in Old Irish, but it has remained to some extent in Scottish Gaelic and traces of it are present in Middle Welsh as well.

Forms that appear in sentence-initial position are called absolute, those that appear after a particle are called conjunct (see Dependent and independent verb forms for details). The paradigm of the presentactiveindicative of the Old Irish verb beirid "carry" is as follows; the conjunct forms are illustrated with the particle ní "not".

Absolute

Conjunct

1st person singular

biru "I carry"

ní biur "I do not carry"

2nd person singular

biri "you carry"

ní bir "you do not carry"

3rd person singular

beirid "s/he carries"

ní beir "she/he does not carry"

1st person plural

bermai "we carry"

ní beram "we do not carry"

2nd person plural

beirthe "you carry"

ní beirid "you do not carry"

3rd person plural

berait "they carry"

ní berat "they do not carry"

In Scottish Gaelic this distinction is still found in certain verb-forms:

Absolute

Conjunct

cuiridh "puts/will put"

cha chuir "doesn't put/will not put"

òlaidh "drinks/will drink"

chan òl "doesn't drink/will not drink"

ceannaichidh "buys/will buy"

cha cheannaich "doesn't buy/will not buy"

In Middle Welsh, the distinction is seen most clearly in proverbs following the formula "X happens, Y does not happen" (Evans 1964: 119):

Pereid y rycheu, ny phara a'e goreu "The furrows last, he who made them lasts not"

The older analysis of the distinction, as reported by Thurneysen (1946, 360 ff.), held that the absolute endings derive from Proto-Indo-European "primary endings" (used in present and future tenses) while the conjunct endings derive from the "secondary endings" (used in past tenses). Thus Old Irish absolute beirid "s/he carries" was thought to be from *bʰereti (compare Sanskritbharati "s/he carries"), while conjunct beir was thought to be from *bʰeret (compare Sanskrit a-bharat "s/he was carrying").

Today, however, most Celticists agree that Cowgill (1975), following an idea present already in Pedersen (1913, 340 ff.), found the correct solution to the origin of the absolute/conjunct distinction: an enclitic particle, reconstructed as *es after consonants and *s after vowels, came in second position in the sentence. If the first word in the sentence was another particle, *(e)s came after that and thus before the verb, but if the verb was the first word in the sentence, *(e)s was cliticized to it. Under this theory, then, Old Irish absolute beirid comes from Proto-Celtic *bereti-s, while conjunct ní beir comes from *nī-s bereti.

The identity of the *(e)s particle remains uncertain. Cowgill suggests it might be a semantically degraded form of *esti "is", while Schrijver (1994) has argued it is derived from the particle *eti "and then", which is attested in Gaulish.

Continental Celtic languages cannot be shown to have any absolute/conjunct distinction. However, they seem to show only SVO and SOV word orders, as in other Indo-European languages. The absolute/conjunct distinction may thus be an artifact of the VSO word order that arose in Insular Celtic.

The concept of the Insular Celtic languages being descended from Hebrew was part of Medieval superstition, but the hypothesis that they had features from an Afro-Asiatic substratum (Iberian and Berber languages) was first proposed by John Morris-Jones in 1900.[3] Some well-known linguists have been adherents such as Julius Pokorny,[4] Heinrich Wagner,[5] and Orin Gensler.[6] There has been further work on the theory by Shisha-Halevy [7] and Theo Vennemann.

"There must have been among Cornish speakers a tendency to a somewhat blurred sound of certain letters, as though there were an obstruction of some sort in their vocal organs, not altogether unlike that attributed on the stage and in fiction, with some foundation in fact, to the Hebrew race. This is shown by the tendency to turn "s" and "z" into "sh" and "zh", and to insert d before n, b before m. "[8]

The theory has been criticised by Graham Isaac[9] and by Kim McCone.[10] Isaac alleges that the 20 points identified by Gensler are trivial, dependencies or vacuous without giving a rationale. Thus, he considers the theory to be not just unproven but also wrong.